1. Field of the Invention
This invention is directed to an aqueous dentifrice composition containing a plurality of reactive agents in which one of the agents is encapsulated in a pressure rupturable shell.
2. The Prior Art
It has long been known to include fluoride containing compounds in dentifrices as anticaries agents, and it has been established that these compounds are effective to reduce the incidence of dental caries. Fluoride compounds which are deemed to be the most effective are sodium fluoride, sodium monoflurophosphate and stannous fluoride. The fluoride compounds are effective mainly due to the fluoride ions which improve the acid resistance of tooth enamel and accelerate recalcification of decayed teeth in their early stage when the decalcification has proceeded only slightly. The effect of improving the acid resistance of the enamel is believed to be due to the fact that the fluoride ions are incorporated into a crystal lattice of hydroxyapatite which is the main constituent of tooth enamel or, in other words, fluoride ions partially fluoridate hydroxyapatite and simultaneously repair the lattice irregularities.
The effectiveness of fluoride treatment in providing acid resistance is dependent upon the amount of fluoride ion which is available for deposition on the enamel being treated. It is, therefore, desirable to formulate dentifrice compositions which provide maximum fluoride ion availability in brushing solutions formed using the dentifrice.
It is known to the art, e.g., U.S. Pat. No. 5,045,305, that an effective way of depositing fluoride on teeth is to use a two-component rinse composition to deposit freshly precipitated calcium fluoride on teeth in which one rinse solution contains CaCl.sub.2 and the other contains fluoride ions in the form of NaF, the separate solutions being admixed immediately prior to introduction in the mouth, to effect interaction and rapid precipitation of CaF.sub.2.
U.S. Pat. No. 5,145,668 discloses a method of fluoridating teeth wherein there is mixed in the mouth a first solution containing a soluble calcium salt such as CaCl.sub.2 contained in a non-reactive vehicle and a second component containing a fluoride compound such as sodium fluorosilicate (Na.sub.2 SiF.sub.6) contained in a non-reactive vehicle, the mixing of the components resulting in the precipitation of calcium fluoride and its deposition on tooth surfaces.
Although the methods disclosed in U.S. Pat. No. 5,045,305 and U.S. Pat. No. 5,145,668 are effective means to achieve fluoridation, the separate solutions containing calcium and fluoride salts must be mixed daily which is a time consuming daily chore. As a result, it is very difficult for the potential beneficiaries of such therapy to faithfully adhere to the regimen. However, simply combining the calcium and fluoride salts into a single formulation will not provide an effective means for fluoridation as the presence of the calcium salt removes soluble ionic fluoride from the dentifrice by forming insoluble and inactive calcium fluoride (CaF.sub.2) thereby reducing the anticariogenic effectiveness of the fluoride dentifrice. Efforts to practice the methods disclosed in these U.S. patents using semi-solid, extrudable vehicle formulations such as toothpastes and gels have been unable to provide the theoretical maximum soluble fluoride because of the tendency for the ionic fluoride originally included in the dentifrice to be prematurely inactivated as the levels of the other conventional dentifrice composition ingredients such as antitartar pyrophosphate salts, antibacterial agents such as triclosan, abrasives such as dicalcium phosphate or silica are increased to that required in the semi-solid dentifrice.
Thus, there is a clear need to formulate a semi-solid dentifrice product such as a toothpaste or gel utilizing a fluoride compound or other active compound wherein the ingredients used to prepare the dentifrice vehicle do not interact with each other such as in the aforementioned inactivation of fluoride ion so that optimum uptake of fluoride is accomplished when the dentifrice is applied to the teeth. Moreover, it is desirable to include reactive compounds such as calcium and fluoride salts in a single highly stable dentifrice form which is susceptible to conventional packaging and dispensing systems and which can be readily and effectively used by the consumer.
The dentifrice art discloses several means to isolate active ingredients from interaction with other ingredients present in the dentifrice. For example, U.S. Pat. Nos. 3,957,964, 3,929,988, 4,071,614 and 4,348,378 disclose aqueous dentifrices containing ingredients such as flavors and dyes whereby such ingredients are encapsulated in rupturable, water-insoluble capsules so that the flavors and dyes are maintained substantially separate from other dentifrice ingredients during manufacture and storage, while subsequently being released when the dentifrice containing the encapsulated ingredients are applied topically to tooth surfaces, the mechanical agitation with a toothbrush rupturing the encapsulating shell whereby the encapsulated ingredient is released to the tooth surface. Materials from which the encapsulating shell is formed are diverse and include synthetic organic plastic materials such as phenol formaldehydes, vinyl chloride polymers, polyethylene, polypropylene, polyurethanes, ABS resins, waxes and cellulosic materials such as ethyl cellulose, butyl cellulose and nitrocellulose.
U.S. Pat. No. 4,837,008 discloses incorporating both bicarbonate and peroxide salts in a non-aqueous dentifrice vehicle whereby interaction between the two salts are prevented by providing at least one of the salts with a water soluble barrier coating of a non-toxic polymer or gum such as a cellulosic polymer or guar gum whereby the salts are released in the aqueous environment of the mouth when the dentifrice is applied to the teeth.
Although the coatings and encapsulants materials of the prior art are effective to some degree to prevent interaction between active compounds such as soluble fluoride with other dentifrice ingredients, it has been determined that such materials are inadequate to fully prevent interaction due to premature leakage of soluble reactive salts from the coating and encapsulating materials whereby, as the case of fluoride salts, there results a significant loss of ionic fluoride and reduced levels of soluble fluoride availability in the dentifrice compositions stored for prolonged period of time.